I'm a bit of a newbie here myself so I hope you don't mind me replying as I think I've just recently figured out what you're looking for....

From your Admin Dashboard, hover over "Posts" and this will reveal four potential dropdowns All Posts, Add New, Categories and Tags. Click on Categories to get to your Categories page. Click on the particular category you are wanting and this will show you an Edit Page with Name, Slug, Parent and Description.

Use the "Description" box to add text to the top of your Category Page. You can use html for paragraphs and the like. Hope I've understood you correctly and this is what you are after.